Adhesive tapes, labels and other constructions are used for many purposes and are made with numerous types of adhesive compositions, including tackified and untackified rubber-based and acrylic-based PSA polymers. To be suitable for gpp or removable, repositionable label applications, a PSA should exhibit adhesion to a variety of paper, film, and metal substrates, good mandrel performance (the ability to remain adhered to a curved surface), and a favorable balance of looptack, peel and shear.
Most rubber-based and acrylic PSAs and PSA constructions are not repulpable, that is, they are not water-dispersible or water-soluble. When a PSA-coated paper label construction is subjected to the repulping (fiber shearing) process, the adhesives tend to agglomerate and form globules, commonly referred to in the trade as "stickies." Stickies adversely affect the quality of recycled paper and can have a deleterious effect on paper making equipment.
Because of a continuing interest in recycling, including recycling of paper, plastics and corrugated board, several attempts have been made to solve the stickies problem, with mixed success. One approach has been to make labels and other adhesive constructions with water-dispersible adhesive microspheres. U.S. Pat. Nos. 3,691,140 (Silver), 4,155,152 (Baker et al.), 4,495,318 (Howard), 4,598,212 (Howard), and 4,810,763 (Mallya et al.) are representative. Although adhesive microspheres disperse in the paper fibers during repulping, they are not removed from the fibers, and may cause problems if the paper fibers are subjected to another round of repulping and recycling.
Water soluble adhesives are available and can be effectively removed from paper fibers during the recycling process, but remain and concentrate in the water system. Such concentration becomes especially problematic in paper recycling mills that reuse all or part of their water supply. Water-soluble materials will eventually precipitate from the water and contaminate the paper pulp and process equipment. This results in cosmetic and functional flaws in the recycled paper product and increased maintenance and downtime to clean the recycling equipment. Further, such adhesives suffer from poor shelf life, poor humidity and/or age performance, and/or a propensity to bleed into the paper facestock (which reduces peel performance). Repulpable emulsion adhesives having a highly polar polymer structure have been developed; unfortunately, they tend to exhibit low adhesion to vinyl (olefinic) substrates, such as HDPE. Repulpable emulsion adhesive comprised of low molecular weight materials have been made; they have exhibited reduced mandrel and shear performance and a tendency to leave an adhesive residue on applied substrates.
Water-dispersible emulsion PSAs also have been made using alkyl phenol ethoxylate (APE)-type surfactants. U.S. Pat. Nos. 5,536,800 (Scholz, et al.) and 5,550,181 (Scholz) are representative. Recently, however, APE-surfactants have come under scrutiny by the environmental and medical communities.
To meet the conflicting demands of the label and tape industry, repulping industry, environmentalists, and consumers, a high-performance, repulpable, gpp or removable, repositionable label adhesive suitable for use in a wide variety of applications is needed.